The invention relates to a coating composition for metal conductors such as, for example, wires, with improved partial discharge resistance and good mechanical properties.
Three-phase a.c. motors, for example, frequency converter-controlled motors or high voltage asynchronous machines require the use of wire wrappings which satisfy the stringent requirements in respect of thermal endurance and the mechanical properties, mainly the flexural strength of the insulation layer, in order to be able to withstand high voltage loads and pulse-shaped voltage loads without damage.
A further requirement in respect of wire wrappings of electrical equipment is the partial discharge resistance of the wire coatings. Adjacent wire wrappings in particular may be exposed to high voltage loads and pulse-shaped voltage loads. For these purposes, the coatings must exhibit a high partial discharge resistance.
According to WO 96/41 909, within the context of a multi-layer coating for wires, a coating composition is used which comprises a binder and a particulate material, wherein the particulate material may be present in the binder in an amount from 1 wt. % to 65 wt. % and may be metal oxides, for example, titanium dioxide, zirconium oxide, zinc oxide, iron oxide or aluminas. The particulate material has no chemical reactivity whatsoever. During the manufacture of such coated wires, preliminary extension may occur which leads to destruction of the coating layers and hence to a drastic decrease in the partial discharge resistance.
Similar compositions with comparable properties are described in DE-A 198 32 186.
According to DE-A 196 50 288, at least one of the electrically insulating coating layers contains an organically modified silica (hetero)polycondensate prepared by hydrolytic condensation of compounds of silicon and optionally of boron, aluminum, phosphorus, tin, lead, the transition metals, lanthanides and actinides, the monomer units being composed essentially of inorganic and organic components which are substantially crosslinked. The coatings obtained have good thermal shock resistance and surface quality. Good flexibilities are not obtained.
In the as yet unpublished German patent application 198 11 333.1 of the same Applicant, a partial discharge-resistant coating is proposed which, in addition to binders, also contains element-organic compounds, particularly of silicon, germanium titanium and zirconium. The organic radicals used are C1 to C20 alkyl radicals or chelating radicals, alkylamine, alkanolamine, acetate, citrate, lactate and/or acetonate radicals. The organometallic compounds used are monomeric compounds.
In the as yet unpublished German application 198 41 977.5, inorganic-organic hybrid polymers are used. The transition from monomeric element-organic compounds to element-organic hybrid polymers leads to a further improvement in the partial discharge resistance of the coating layer in question.
For stringent requirements, particularly for three-phase a.c. motors in continuous operation and frequency converter-controlled motors, the partial discharge resistance obtained is still in need of improvement.
The object of the present invention is, therefore, to provide a coating composition for metal conductors, particularly wires, the partial discharge resistance of which is increased compared with the solutions of the prior art, particularly when the coated wire is extended. Moreover, the applicability of the coating composition as a single-layer application or as a coating in a multi-layer application, and the surface quality and flexibility of the coating should be improved.